A conventional laser printer comprises a main controller forming a picture image data based on a data from an external host computer, and a mechanical element controller a controlling mechanical elements (hereinafter mechanisms). In this laser printer a sheet of paper is supplied into the printer from a paper supply tray through a paper supply port of the printer. At this time, jamming of paper sheets during paper supplying operation is supervised by a feed paper sensor installed at the paper supply port. Upon detection of jamming an information meaning the generation of paper jamming is stored as an engine status in a mechanism controller in the printer.
Sheets of paper whose printings have been completed are discharged onto paper receiving tray through a paper discharge port of the printer. During this time, supervision of paper jamming in the main body of the printer and in the paper discharge port are performed by a discharged paper sensor provided at the paper discharge port. Where paper jamming is detected, an information informing generation of an engine status in a mechanism controller.
According to the prior art technique the main controller contains therein a timer for anticipating an interval in which a sequence is being executed and a time at which the discharge of paper sheets is terminated by taking a time at which feeding of the sheets of paper has started as a reference point. During this period, the main controller inquires the mechanism controller as to the engine status. This is made for the purpose of judging whether printing data for one sheet of paper is to be held until what time, in addition to the supervision of the paper jamming. In this manner, the fact that time supervision of the printing sequence is effected in the main controller complicates the processing in the main controller. When the type of mechanisms is changed, the printing process speed and the printing sequence change so that it is necessary to amend the printing sequence time supervisory software of the main controller, thus degrading the design efficiency. Accordingly, it is very difficult to obtain a system by arbitrary combining mechanisms having different printing process speeds and printing sequence with a controller having a data processing speed commensurate with cost. To satisfy such requirement, it is necessary to add considerable amount of softwares, resulting in a cost up.